1. Field of the Invention
The present invention relates to mass spectrometers and methods of mass spectrometry.
2. Discussion of the Prior Art
Ion guides comprising rf-only multipole rod sets such as quadrupoles, hexapoles and octopoles are well known.
An alternative type of ion guide known as an “ion funnel” has recently been proposed by Smith and co-workers at Pacific Northwest National Laboratory. An ion funnel comprises a stack of ring electrodes of constant external diameter but which have progressively smaller internal apertures. A dc voltage/potential gradient is applied along the length of the ion guide in order to urge ions through the ion funnel which would otherwise act as an ion mirror.
A variant of the standard ion funnel arrangement is disclosed in Anal. Chem. 2000, 72, 2247-2255 and comprises an initial drift section comprising ring electrodes having constant internal diameters and a funnel section comprising ring electrodes having uniformly decreasing internal diameters. A dc voltage gradient is applied across both sections in order to urge ions through the ion funnel.
Ion funnels have not been successfully employed in commercial mass spectrometers to date.
One reason for this may be that ion funnels suffer from a narrow bandpass transmission efficiency i.e. the ion funnel may, for example, only efficiently transmit ions having mass to charge ratios (“m/z”) falling within a narrow range e.g. 100<m/z<200. Reference is made, for example, to FIGS. 5A and 5B of Anal. Chem. 1998, 70, 4111-4119 wherein experimental results are presented comparing observed mass spectra obtained using an ion funnel with that obtained using a conventional ion guide. The experimental results show that both relatively low m/z and relatively high m/z ions fail to be transmitted by the ion funnel. Reference is also made to pages 2249 and 2250 of Anal. Chem 2000, 72, 2247-2255 which similarly recognises that ion funnels suffer from an undesirably narrow m/z transmission window.
Another reason may be that ion funnel ion guides require both an rf voltage and a dc voltage gradient to be applied to the ring electrodes. However, the design and manufacture of a reliable power supply capable of supplying both an rf voltage and a dc voltage gradient which is decoupled from the rf voltage is a non-trivial matter and increases the overall manufacturing cost of the mass spectrometer.
It is therefore desired to provide an improved ion guide.